An optical switching system employing an electro-optic device which exhibits the effects of birefringence, such as a liquid crystal cell, does not transmit light of uniform intensity when the system output is observed from different locations along a viewing cone defined at an angle measured relative to the axis or path of the transmitted light. The nonuniform intensity of light changes the contrast of the display and is caused by the spurious transmission of contaminant light whose intensity varies as a function of azimuthal angle along the viewing cone. In the case of an optical switching system which develops two optical transmission states of light of different colors, the effect of birefringence is that light rays exiting the system in either one of its optical transmission states includes contaminant light rays of the color of the other optical transmission state which vary in intensity at different locations along the viewing cone.
There have been known heretofore methods and apparatus for improving the off-axis viewing angle performance of optical switches incorporating electro-optic devices which comprise birefringent materials. One class of patents discloses methods and apparatus for improving the viewing angle characteristics of twisted nematic liquid crystal display devices.
In particular, U.S. Pat. No. 4,385,806 of Fergason discloses the introduction into a twisted nematic liquid crystal assembly of at least two retardation plate devices arranged in a particular manner to improve the off-axis viewing angle performance of the device.
U.S. Pat. No. 4,192,060 of Washizuka et al. discloses a twisted nematic liquid crystal cell whose transparent electrodes have undergone a horizontal orientation rubbing process to obtain a preferred director alignment and thereby improve the viewing angle characteristics of the liquid crystal cell.
U.S. Pat. No. 4,183,630 of Funada et al. discloses the use of a fiber plate which includes a plurality of optical fibers positioned adjacent the outer surface of one of a pair of substrates which form an inclusion for a layer of twisted nematic liquid crystal material. The use of the fiber plate is said to preserve the uniformity of display contrast as the voltage applied to the liquid crystal cell is reduced.
U.S. Pat. No. 4,120,567 of Goodman et al. discloses the method for increasing the viewing angle of a twisted nematic liquid crystal cell by increasing the ratio of the voltage applied to the cell to its threshold voltage.
U.S. Pat. No. 3,966,305 of Young discloses the deposition of a dielectric layer between the patterned conductive layer and the director alignment film of a twisted nematic liquid crystal display device in an attempt to improve the viewing angle of the display.
A second class of patents discloses techniques directed to either filtering undesirable ambient light or introducing a source of back-lighting to enhance display contrast and thereby provide a more desirable viewing angle.
In particular, U.S. Pat. No. 3,887,791 of Kitchens discloses the use of a prism overlaying display cells positioned in a casing to alter the angle of light transmission of the display images and thereby provide an improved viewing angle. The use of a prism, however, only shifts the angle of transmission of the display and filters undesirable ambient light. The prism does not correct for nonuniform contrast as the display is observed from different azimuthal angles of a particular cone of view.
U.S. Pat. No. 3,869,195 of Aldrich et al. discloses the use of segmented back-lighting of a liquid crystal display as a means to improve the viewing angle thereof. The source of segmented back-lighting is an electroluminescent layer in which one transparent electrode applied to the electroluminescent material is segmented to coincide with the segmentation of the liquid crystal panel. The effect is to provide high intensity contrast between the activated display segments and the nonactivated areas of the display over a wide range of ambient light conditions.
A third class of patents discloses the use of ancillary optical components with optical switching systems to enhance display contrast.
Included in this class of patents is U.S. Pat. No. 4,088,400 of Assouline et al. which discloses an electro-optical display device having positioned adjacent its exit polarizer a diffuser element in association with an additional polarizer to improve the viewing angle of the display without loss of contrast. A pair of quarter-wave plates is disposed on opposite sides of the diffuser element to eliminate back-scattering of ambient light to the observer's eye. The method of Assouline et al. appears to be inapplicable for increasing the contrast of reflection-type liquid crystal displays.
U.S. Pat. No. 3,838,906 of Kumada discloses an optical switch comprising an electro-optic crystal and a birefringent crystal arranged in cascade in a manner such that the sign of the birefringence of the former is opposite to that of the latter. Kumada states that this arrangement of electro-optic devices provides an optical switch which can effectively block in its opaque output state light having an incident angle as great as 30.degree..